The principal objective of this study will be to analyze the method of meiotic restitution by which a constant chromosome number is maintained by representatives of the various parthenogenetic vertebrates, and to attempt to understand the origin of parthenogenesis in this group. This study will consist of three parts. The first will comprise a continuation of my previous investigations with the triploid parthenogenetic lizard Cnemidophorus uniparens, and includes further analysis of certain critical meiotic events that were not adequately documented earlier, and an analysis of the phase in oogenesis when the proposed premeiotic doubling of the chromosomes presumably occurs. The second part will attempt to determine if more than one pattern of meiotic restitution exists among the various phyletic groups of parthenogenetic vertebrates. Here, a comparison of meiosis will be made between gynogenetic and spontaneous induced parthenogenetic types, between species with different ploidies, between gynogenetic fish and amphibians, and between species of lizards belonging to different families. Finally, the third objective will consist of an analysis of the extent and frequency of tychoparthenogenesis in unfertilized eggs of bisexual species, closely related to the unisexual ones and suspected of having been involved in their origin, in an effort to understand whether parthenogenesis evolved spontaneously or via hybridization.